1. Cooling Capacity: A 3-ton air conditioner has a cooling capacity of approximately 36,000 British Thermal Units per hour (BTU/hr).
2. CFM (Cubic Feet per Minute): The air return duct should be sized to handle the airflow required for the unit's cooling capacity. A general guideline is to provide a minimum airflow rate of 400 cubic feet per minute (CFM) per ton of cooling. For a 3-ton unit, the recommended airflow rate would be around 1200 CFM.
3. Duct Sizing Calculation: To calculate the duct size, you can use the following formula:
Duct Size (in inches) = √[(CFM x 144) / (Velocity in feet per minute)]
Where:
- CFM = 1200 CFM (for a 3-ton unit)
- 144 = Conversion factor from square feet to square inches
- Velocity = Recommended velocity of airflow through the duct
The recommended velocity for cold air return ducts is typically between 350 and 550 feet per minute (FPM). For this calculation, we'll use a velocity of 450 FPM.
√[(1200 x 144) / 450] ≈ 14.14
Therefore, the recommended duct size for a cold air return for a 3-ton unit is approximately 14 inches in diameter or a rectangular duct with equivalent cross-sectional area.
It's important to note that these calculations are based on general guidelines and may vary depending on specific system design and installation factors. Consulting with an HVAC professional is recommended to determine the exact duct sizing and layout for your specific application.
